Battery testing

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Posts: 9
Joined: 17 Jul 2022, 00:16

Battery testing

Post by mojoe »

While awaiting my radio, I bought three 18650 cells to use in the optional 3D printed battery pack. I bought Samsung INR18650-25R cells, which are nominally rated 3.6 V, 2500 mAh. These are flat top, unprotected cells. I wanted to use protected cells, but they are longer, and won't fit the battery holder.

I tested each cell individually, using my CBA IV Battery Analyzer. Each cell tested very much the same, so the results can be multiplied by three, to apply to the entire battery pack when assembled.

I ran a simulated usage test of TX and RX, repeated until the cell voltage dropped to 3.0 V on TX. Although the battery analyzer measures voltage to the millivolt, my reported results below show some slight rounding to a tenth of a Volt.

Based on the typical current draws @ 13.8 V as listed on the DL2MAN website, I used higher current draws for the test, to give me the same wattages. This was based on a voltage of 10.8 V (3.6 V nominal per cell).

Now that I think about it, since the website listed values were obtained using a steady supply voltage of 13.8 V, and a battery voltage varies, I should have run the test using a constant wattage, not constant current draw. Maybe I'll do this after I receive my radio, to see if the results are substantially different.

I started the current draw test with 30 seconds of TX, followed by 30 seconds of RX. This is repeated until the TX voltage drops to 3.0 V. You will note in the attached graphs that the RX voltage stays 0.1 V above the TX voltage across the entire test. This indicates the fast recovery of the cell at these low current draws.

Attached are three graphs. One shows a closeup of the first 60 minutes of the test, where the voltage stays very close to 4.0 V. The complete test graph shows a fairly linear voltage drop from 60-369 minutes, then a rapid drop off. The steep drop in voltage can be seen in the last closeup graph, and is typical of lithium cells.

Here are some times and voltages for the TX cycles:

Start: 4.0 V x 3 = 12.0 V
112 min: 3.8 V x 3 = 11.4 V
200 min: 3.6 V x 3 = 10.8 V
308 min: 3.4 V x 3 = 10.2 V
369 min: 3.2 V x 3 = 9.6 V
384 min: 3.0 V x 3 = 9.0 V

Using these cells in a battery pack would give over six hours of constant use, transmitting and receiving at 30 second intervals. This would be much worse than any normal use, but gives a benchmark to use to gauge your usage. Like I said, I should do another test at a later time, based on wattage, to see how the results differ.
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